1. Field of Invention
The present invention relates to an ion source for use in semiconductor fabrication and more particularly to an ion source configured with its cathode located external to the ionization chamber, the ion source being operable in a first mode of operation or may be configured as a dual mode ion source that is selectively operable in both a first mode of operation, such as an arc discharge mode and a second mode of operation, such as a direct electron impact mode of operation, with a single electron emitter that can be used for ionizing gases and vapors and producing monatomic ions in an arc discharge mode of operation and molecular ions, such as cluster ions in a direct electron impact mode of operation.
2. Description of the Prior Art
Electron emitters (also known as electron guns) with both directly heated and indirectly heated cathodes (IHC) are known in the art to be used in ion implantation systems. In known ion implantation systems, electron emitters with such heated and indirectly heated cathodes are normally disposed within an ionization chamber and are therefore subject to a relatively harsh environment as a result of the plasma developed within the ionization chamber. Electron emitters with IHCs offer advantages over those with directly heated cathodes in reliability and life time. In particular, emitters with directly heated cathodes include a relatively small wire that forms a filament. These filaments are known to fail in such harsh environments in a relatively short time. On the other hand, electron emitters which include Indirectly heated cathodes include a relatively massive cathode that is heated indirectly by electron bombardment from a filament. In the case of the IHC electron emitters, the cathode emits electrons thermionically. Although, the IHC is exposed to a harsh plasma environment, the massive cathode having a substantially larger mass than the filament of a directly heated cathode provides a relatively longer life than such directly heated cathodes.
As such, IHC electron emitters are typically used to ionize feed materials in hot arc discharge mode, where the IHC emitter sits inside the ionization chamber. The filament inside the IHC electron emitter is heated with electrical current, and biased into negative potential with respect to the solid emitter block. This allows electrons from the filament to be accelerated into the solid emitter heating it up. The emitter block in turn is biased negatively with respect to the ion source body. When the emitter reaches sufficient temperature, it will start emitting electrons igniting arc discharge between the emitter and the source wall forming plasma.
In order to further increase the operating lifetime of such IHCs, devices have been developed which include a cathode and a filament in which the cathode is located inside the ionization or arc chamber of the ion implantation system while the filament is located outside the ionization chamber, as described in detail in Varian U.S. Pat. No. 7,138,768. With such a configuration, by locating the filament outside the harsh environment of the ionization chamber, the operating lifetime of such IHCs is relatively longer than IHCs which are totally located within the ionization chamber.
In recent development of cluster ion sources, heat management has become increasingly important, as many of the cluster molecules are quite fragile and can be dissociated when exposed to hot surfaces like the IHC emitter. It is thus beneficial to remove the electron emitter form the source and situate it externally, namely, outside the ionization chamber, for example, as disclosed in SemEquip U.S. Pat. No. 7,185,602, hereby incorporated by reference. Also the significant material deposits that some of the cluster materials produce favor the removal of the emitter from the source, as there is a risk of building up flakes that will short the IHC to the ionization chamber potential.
This will change the operational parameters of the emitter somewhat. Now the emitter is removed from the higher gas pressure of the ionization chamber into the usually pumped source housing, where the gas pressure can be an order of magnitude or more lower, striking an arc discharge between the IHC emitter and the source will become more difficult. If plasma is ignited, there will be significant diffusion losses before the bulk of the plasma would get into the ionization chamber. For cluster ionization, the required extracted beam current densities are typically an order of magnitude lower than for traditional atomic implant species. This means that dense plasma is not needed in order to create sufficient ion beam currents. Externally located electron gun which forms an electron beam either from a directly or indirectly heated emitter will be able to inject sufficient electron current into the ionization chamber.
To form an electron beam that can be efficiently injected into the source, i.e, ionization chamber, electron optics are normally used to pull and focus the beam. Typically this means placing an anode electrode between the source and emitter. The electrons are pulled from the emitter with the assistance of the anode potential and accelerated to energy e*Vcathode+e*Vanode going across the emitter-anode gap and through the anode. At the anode-source gap the electron beam is decelerated to e*Vcath and focused by the decel lens effect. This setup will allow for refined tuning of the electron beam current, size and emittance. The downside is that the distance the electrons are traveling will increase by the extent of the anode and anode gap. Typically the anode voltages are higher than the cathode voltage. This can lead into voltage holding issues.
Ion beams are produced from ions extracted from an ion source. An ion source typically employs an ionization chamber connected to a high voltage power supply. The ionization chamber is associated with a source of ionizing energy, such as an arc discharge, energetic electrons from an electron-emitting cathode, or a radio frequency or microwave antenna, for example. A source of desired ion species is introduced into the ionization chamber as a feed material in gaseous or vaporized form where it is exposed to the ionizing energy. Extraction of resultant ions from the chamber through an extraction aperture is based on the electric charge of the ions. An extraction electrode is situated outside of the ionization chamber, aligned with the extraction aperture, and at a voltage below that of the ionization chamber. The electrode draws the ions out, typically forming an ion beam. Depending upon desired use, the beam of ions may be mass analyzed for establishing mass and energy purity, accelerated, focused and subjected to scanning forces. The beam is then transported to its point of use, for example into a processing chamber. As the result of the precise energy qualities of the ion beam, its ions may be implanted with high accuracy at desired depth into semiconductor substrates.
The Ion Implantation Process
The conventional method of introducing a dopant element into a semiconductor wafer is by introduction of a controlled energy ion beam for ion implantation. This introduces desired impurity species into the material of the semiconductor substrate to form doped (or “impurity”) regions at desired depth. The impurity elements are selected to bond with the semiconductor material to create electrical carriers, thus altering the electrical conductivity of the semiconductor material. The electrical carriers can either be electrons (generated by N-type dopants) or “holes” (i.e., the absence of an electron), generated by P-type dopants. The concentration of dopant impurities so introduced determines the electrical conductivity of the doped region. Many such N- and P-type impurity regions must be created to form transistor structures, isolation structures and other such electronic structures, which collectively function as a semiconductor device.
To produce an ion beam for ion implantation, a gas or vapor feed material is selected to contain the desired dopant element. The gas or vapor is introduced into the evacuated high voltage ionization chamber while energy is introduced to ionize it. This creates ions which contain the dopant element (for example, in silicon the elements As, P, and Sb are donors or N-type dopants, while B and In are acceptors or P-type dopants). An accelerating electric field is provided by the extraction electrode to extract and accelerate the typically positively charged ions out of the ionization chamber, creating the desired ion beam. When high purity is required, the beam is transported through mass analysis to select the species to be implanted, as is known in the art. The ion beam is ultimately transported to a processing chamber for implantation into the semiconductor wafer.
Similar technology is used in the fabrication of flat-panel displays (FPD's) which incorporate on-substrate driver circuitry to operate the thin-film transistors which populate the displays. The substrate in this case is a transparent panel such as glass to which a semiconductor layer has been applied. Ion sources used in the manufacturing of FPD's are typically physically large, to create large-area ion beams of boron, phosphorus and arsenic-containing materials, for example, which are directed into a chamber containing the substrate to be implanted. Most FPD implanters do not mass-analyze the ion beam prior to its reaching the substrate.
Many ion sources used in ion implanters for device wafer manufacturing are “hot” sources, that is, they operate by sustaining an arc discharge and generating a dense plasma; the ionization chamber of such a “hot” source can reach an operating temperature of 800 C or higher, in many cases substantially reducing the accumulation of solid deposits. In addition, the use of BF.sub.3 in such sources to generate boron-containing ion beams further reduces deposits, since in the generation of a BF.sub.3 plasma, copious amounts of fluorine ions are generated; fluorine can etch the walls of the ion source, and in particular, recover deposited boron through the chemical production of gaseous BF.sub.3. With other feed materials, however, detrimental deposits have formed in hot ion sources. Examples include antimony (Sb) metal, and solid indium (In), the ions of which are used for doping silicon substrates.
A typical commercial ion implanter is shown in schematic in FIG. 1. The ion beam I is shows propagating from the ion source 42 through a transport (i.e. “analyzer”) magnet 43, where it is separated along the dispersive (lateral) plane according to the mass-to-charge ratio of the ions. A portion of the beam is focused by the magnet 43 onto a mass resolving aperture 44. The aperture size (lateral dimension) determines which mass-to-charge ratio ion passes downstream, to ultimately impact the target wafer 55, which typically may be mounted on a spinning disk 45. The smaller the mass resolving aperture 44, the higher the resolving power R of the implanter, where R=M/.DELTA.M (M being the nominal mass-to-charge ratio of the ion and .DELTA.M being the range of mass-to-charge ratios passed by the aperture 44). The beam current passing aperture 44 can be monitored by a moveable Faraday detector 46, whereas a portion of the beam current reaching the wafer position can be monitored by a second Faraday detector 47 located behind the disk 45. The ion source 42 is biased to high voltage and receives gas distribution and power through feedthroughs 48. The source housing 49 is kept at high vacuum by source pump 50, while the downstream portion of the implanter is likewise kept at high vacuum by chamber pump 51. The ion source 42 is electrically isolated from the source housing 49 by dielectric bushing 52. The ion beam is extracted from the ion source 42 and accelerated by an extraction electrode 53. In the simplest case (where the source housing 49, implanter magnet 43, and disk 45 are maintained at ground potential), the final electrode of the extraction electrode 53 is at ground potential and the ion source is floated to a positive voltage V.sub.a, where the beam energy E=qV.sub.a and q is the electric charge per ion. In this case, the ion beam impacts the wafer 55 with ion energy E. In other implanters, as in serial implanters, the ion beam is scanned across a wafer by an electrostatic or electromagnetic scanner, with either a mechanical scan system to move the wafer or another such electrostatic or electromagnetic scanner being employed to accomplish scanning in the orthogonal direction.
As shown, in FIG. 2, a Bernas ion source a is mounted to the vacuum system of the ion implanter through a mounting flange b which also accommodates vacuum feedthroughs for cooling water, thermocouples, feed material as a dopant gas feed, N2 cooling gas, and power. The gas feed c feeds gas into the arc chamber d in which the gas is ionized. Also provided are dual vaporizer ovens e, f in which solid feed materials such as As, Sb2O3, and P may be vaporized. The ovens, gas feed, and cooling lines are contained within a cooled machined aluminum block g. The water cooling is required to limit the temperature excursion of the aluminum block g while the vaporizers, which operate between 100 C. and 800 C., are active, and also to counteract radiative heating by the arc chamber d when the source is active. The arc chamber d is mounted to the aluminum block g.
The gas introduced to arc chamber d is ionized through electron impact with the electron current, or arc, discharged between the cathode h and the arc chamber d. To increase ionization efficiency, a uniform magnetic field i is established along the axis joining the cathode h and an anticathode j by external Helmholz coils, to provide confinement of the arc electrons. An anticathode j (located within the arc chamber d but at the end opposite the cathode h) is typically held at the same electric potential as the cathode h, and serves to reflect the arc electrons confined by the magnetic field i back toward the cathode h and back again repeatedly. The trajectory of the thus-confined electrons is helical, resulting in a cylindrical plasma column between the cathode h and anticathode j. The plasma density within the plasma column is typically high, on the order of 1012 per cubic centimeter; this enables further ionizations of the neutral and ionized components within the plasma column by charge-exchange interactions, and also allows for the production of a high current density of extracted ions. The ion source a is held at a potential above ground (i.e., the silicon wafer potential) equal to the accelerating voltage Va of the ion implanter: the energy of the ions E as they impact the wafer substrate is given by E=qVa, where q is the electric charge per ion.
The cathode h is typically a hot filament or indirectly-heated cathode, which thermionically emits electrons when heated by an external power supply. It and the anticathode are typically held at a voltage Vc between 60V and 150V below the potential of the ion source Va. High discharge currents D can be obtained by this approach, up to 7 A. Once an arc discharge plasma is initiated, the plasma develops a sheath adjacent to the surface of the cathode h (since the cathode h is immersed within the arc chamber and is thus in contact with the resulting plasma). This sheath provides a high electric field to efficiently extract the thermionic electron current for the arc; high discharge currents can be obtained by this method.
If the solid source vaporizer ovens e or f are used, the vaporized material feeds into the arc chamber d through vaporizer feeds k and l, and into plenums m and n. The plenums serve to diffuse the vaporized material into the arc chamber d, and are at about the same temperature as the arc chamber d. In this case a co-gas could be introduced either via tube c into chamber d if the co-gas was from a gaseous stock, It would also be possible to utilize whichever solid vaporizer (e or f) was not in use for, the primary feedstock to generate a co-gas from an appropriate solid material.
Cold ion sources, for example the RF bucket-type ion source which uses an immersed RF antenna to excite the source plasma (see, for example, Leung et al., U.S. Pat. No. 6,094,012, herein incorporated by reference), are used in applications where either the design of the ion source includes permanent magnets which must be kept below their Curie temperature, or the ion source is designed to use thermally-sensitive feed materials which break down if exposed to hot surfaces, or where both of these conditions exist. Cold ion sources suffer more from the deposition of feed materials than do hot sources. The use of halogenated feed materials for producing dopants may help reduce deposits to some extent, however, in certain cases, non-halogen feed materials such as hydrides are preferred over halogenated compounds. For non-halogen applications, ion source feed materials such as gaseous B.sub.2H.sub.6, AsH.sub.3, and PH.sub.3 are used. In some cases, elemental As and P are used, in vaporized form. The use of these gases and vapors in cold ion sources has resulted in significant materials deposition and has required the ion source to be removed and cleaned, sometimes frequently. Cold ion sources which use B.sub.2H.sub.6 and PH.sub.3 are in common use today in FPD implantation tools. These ion sources suffer from cross-contamination (between N- and P-type dopants) and also from particle formation due to the presence of deposits. When transported to the substrate, particles negatively impact yield. Cross-contamination effects have historically forced FPD manufacturers to use dedicated ion implanters, one for N-type ions, and one for P-type ions, which has severely affected cost of ownership.
Recently, cluster implantation ion sources have been introduced into the equipment market (see for example, U.S. Pat. Nos. 6,107,634; 6,288,403; and 6,958,481). U.S. Pat. Nos. 6,452,338; 6,686,595; and. 6,744,214, hereby incorporated by reference, disclose ion sources that are unlike the Bernas-style sources in that they have been designed to produce “clusters”, or conglomerates of dopant atoms in molecular form, including ions of the form Asn+, Pn+, or BnHm+, where n and m are integers, and 2≦n≦18. Such ionized clusters can be implanted much closer to the surface of the silicon substrate and at higher doses relative to their monomer (n=1) counterparts, and are therefore of great interest for forming ultra-shallow p-n transistor junctions, for example in transistor devices of the 65 nm, 45 nm, or 32 nm generations. These cluster sources preserve the parent molecules of the feed gases and vapors introduced into the ion source. The most successful of these have used electron-impact ionization, and do not produce dense plasmas, but rather generate low ion densities at least 100 times smaller than produced by conventional Bernas sources, for example, as disclosed in the cluster ion sources mentioned above The use of B18H22 as an implant material for ion implantation of B18Hx+ in making PMOS devices is disclosed in Horsky et al. U.S. Patent Application Publication No. US 2004/0002202 A1, hereby incorporated by reference.
FIG. 3 shows in schematic a cluster ion source 1 as described in more detail in U.S. Pat. No. 6,452,338, hereby incorporated by reference. The vaporizer 2 is attached to the vaporizer valve 3 through an annular metal gasket 4. The vaporizer valve 3 is likewise attached to the ionization chamber 5 by a second annular metal gasket 6. This ensures good thermal conduction between the vaporizer, vaporizer valve, and ionization chamber 5 through intimate contact via thermally conductive elements. A mounting flange 7 attached to the ionization chamber 5 allows mounting of the ion source 1 to the vacuum housing of an ion implanter, and contains electrical feedthroughs (not shown) to power the ion source, and water cooling feedthroughs 8, 9 to cool the ion source. The water feedthroughs 8, 9 circulate water through the source shield 10 to cool the source shield 10 and cool the attached components, the beam dump 11 and electron gun 12 (further described below). The exit aperture 13 is mounted to the ionization chamber 5 face by metal screws (not shown). Thermal conduction of the exit aperture 13 to the ionization chamber 5 is aided by an annular seal 14 which can be made from metal or a thermally conductive polymer.
When the vaporizer valve 3 is in the open position, vaporized gases from the vaporizer 2 can flow through the vaporizer valve 3 to inlet channel 15 into the open volume of the ionization chamber 5. These gases are ionized by interaction with the electron beam transported from the electron gun 12 to the beam dump 11. The ions can then exit the ion source from the exit aperture 13, where they are collected and transported by the ion optics of the ion implanter.
The vaporizer 2 is made of machined aluminum, and houses a water bath 17 which surrounds a crucible 18, wherein resides solid feed materials 19. The water bath 17 is heated by a resistive heater plate 20 and cooled by a heat exchanger coil 21 to keep the water bath at the desired temperature. The heat exchanger coil 21 is cooled by de-ionized water provided by water inlet 22 and water outlet 23. Although the temperature difference between the heating and cooling elements provides convective mixing of the water, a magnetic paddle stirrer 24 continuously stirs the water bath 17 while the vaporizer is in operation. A thermocouple 25 continually monitors the temperature of the crucible 18 to provide temperature feedback for a PID vaporizer temperature controller (not shown). The ionization chamber 5 is made of aluminum, graphite, or molybdenum, and operates near the temperature of the vaporizer 2 through thermal conduction. In addition to low-temperature vaporized solids, the ion source can receive gases through gas feed 26, which feeds directly into the open volume of the ionization chamber 16 by an inlet channel 27. In prior art systems, when the gas feed 26 is used to input feed gases, the vaporizer valve 3 is closed, however, in an embodiment of the invention, the feed material is vaporized in the vaporizer 2 and provided to the chamber 5 as a gas and the co-material gas is also provided to the chamber 5 via gas feed 26. The co-gas is introduced and metered via a commercial mass-flow-controller.
FIG. 4 illustrates the geometry of the ion source with the exit aperture removed; the ion beam axis points out of the plane of the paper. An electron beam 32 is emitted from the cathode 33 and focused by the electron optics 34 to form a wide beam. The electron beam may be asymmetric, in that it is wider perpendicular to the ion beam axis than it is along that axis. The distribution of ions created by neutral gas interaction with the electron beam roughly corresponds to the profile of the electron beam. Since the exit aperture 13 is a wide, rectangular aperture, the distribution of ions created adjacent to the aperture 13 should be uniform. Also, in the ionization of decaborane and other large molecules, it is important to maintain a low plasma density in the ion source This limits the charge-exchange interactions between the ions which can cause loss of the ions of interest. Since the ions are generated in a widely distributed electron beam, this will reduce the local plasma density relative to other conventional ion sources known in the art. The electron beam passes through an entrance channel 35 in the ionization chamber and interacts with the neutral gas within the open volume 16. It then passes through an exit channel 36 in the ionization chamber and is intercepted by the beam dump 11, which is mounted onto the water-cooled source shield 10. Since the heat load generated by the hot cathode 33 and the heat load generated by impact of the electron beam 32 with the beam dump 11 is substantial, these elements, as well as the electron optics or anodes 34, are kept outside of the ionization chamber open volume 16 where they cannot cause dissociation of the neutral gas molecules and ions.
Borohydrides
Borohydride materials such as B.sub.10H.sub.14 (decaborane) and B.sub.18H.sub.22 (octadecaborane) under the right conditions form the ions B.sub.10H.sub.x.sup.+, B.sub.10H.sub.x.sup.−, B.sub.18H.sub.x.sup.+, and B.sub.18H.sub.x.sup.−. When implanted, these ions enable very shallow, high dose P-type implants for shallow junction formation in CMOS manufacturing. Since these materials are solid at room temperature, they must be vaporized and the vapor introduced into the ion source for ionization. They are low-temperature materials (e.g., decaborane melts at 100 C, and has a vapor pressure of approximately 0.2 Torr at room temperature; also, decaborane dissociates above 350 C), and hence must be used in a cold ion source. They are fragile molecules which are easily dissociated, for example, in hot plasma sources.
Contamination Issues of Borohydrides
Boron hydrides such as decaborane and octadecaborane present a severe deposition problem when used to produce ion beams, due to their propensity for readily dissociating within the ion source. Use of these materials in Bernas-style arc discharge ion sources and also in electron-impact (“soft”) ionization sources, have confirmed that boron-containing deposits accumulate within the ion sources at a substantial rate. Indeed, up to half of the borohydride vapor introduced into the source may stay in the ion source as dissociated, condensed material. Eventually, depending on the design of the ion source, the buildup of condensed material interferes with the operation of the source and necessitates removal and cleaning of the ion source.
Contamination of the extraction electrode has also been a problem when using these materials. Both direct ion beam strike and condensed vapor can form layers that degrade operation of the ion beam formation optics, since these boron-containing layers appear to be electrically insulating. Once an electrically insulating layer is deposited, it accumulates electrical charge and creates vacuum discharges, or so-called “glitches”, upon breakdown. Such instabilities affect the precision quality of the ion beam and can contribute to the creation of contaminating particles.
It is desirable at times to be able to run an ion implantation system for implanting either monatomic ions or molecular ions, such as cluster ions. U.S. Pat. No. 7,107,929 and US Patent Application Publication No. 2007/0108394 A1, assigned to the same assignee as the present invention, are examples of ion sources that are configured to operate in dual modes and generate monatomic ions in an arc discharge mode of operation and molecular ions, such as cluster ions in a direct electron impact mode of operation. Some known dual mode ion sources are known utilize a first electron emitter which includes a cathode and an associated anode, remote from the ionization chamber which are configured to operate in a direct electron impact mode for the cluster ion feed material and also employ a second electron emitter disposed within the ionization chamber, to ionize the monatomic ion feed material. These electron emitters either use small gaps around the cathode (with remote support insulators) or adjacent insulators to prevent the cathode from shorting out to the ionization chamber. While such dual mode ion sources are a significant improvement over single mode ion sources, the need for multiple electron emitters in a single ion source adds a substantial amount of complexity.
Thus, there is a need to provide an ion implantation system which includes an ion source which includes a single electron emitter, such as an electron emitter that includes a cathode that is located external to the ionization chamber which eliminates the need for an associated anode and which can be used in an arc discharge mode or alternatively incorporated into a dual mode ion source operable in a direct electron impact mode and an arc discharge mode.